Transcript Maxima and Minima - Glassboro Public Schools

Maxima and Minima
Summary of Maximum and Minimum
Introduction
1) Local maximums and minimums of a function,
f(x), often (but not always) occur at stationary
points where the function's derivative, f'(x), is
zero.
2) You can determine whether a stationary point is
a maximum or a minimum by using the
following rule: If the second derivative of f(x),
that is, f"(x) is negative at the stationary point,
then that stationary point is a maximum. If the
second derivative is positive at the stationary
point, then the stationary point is a minimum.
3) You can solve for the x-values of stationary
points by setting the derivative to zero and
finding what x-values satisfy the resulting
equation.
4) You can find the y-values of the stationary points
by substituting the x-values you got from point
3) into f(x).
Examples:
1)
Let f(x) = -x2 + 3x - 2.
a)
b)
Find its stationary point.
Find f"(x) for the above function and use it
to discover whether the stationary point is
a maximum or a minimum.
2. Let f(x) = x3 - 3x + 1.
a)
b)
Find its stationary points
Find f"(x) for the above function and use it
to discover the nature of the stationary
points are they maximums, minimums, or a
mixed bag, and if they are mixed, which is
which?)
3.
Let f(x) = 1/(x2 - 2x + 2)
a) Find f"(x) and use it to discover whether the stationary
point is a maximum or minimum.
4. No maxima-minima problems would be complete without
an economics problems. Suppose you are in the business of
manufacturing and selling bow-ties. Your marketing
research indicates that the number of bow-ties you will sell
each month is related to the price you charge, x,
according to the function: n(x) = 40000 - 800x. This is called
the demand function. So, for example, if you gave them
away, you would only get rid of 40,000 bow-ties per month.
At the opposite end, if you charged $50 apiece for them,
you would sell none. You are looking for a happy medium
in between where you will earn some profit. It turns out it
costs you $1000 per month to rent the building where you
have your bow-tie factory. It also costs you $2 per bow-tie
in material and labor to produce them. If profit is revenue
minus cost, determine the price, x, you should charge in
order to maximize your profit.
5. Three sides of a window frame form three of the sides of a
rectangle. The fourth side is a semicircle. The perimeter of
this shape is fixed at 3 meters. Find the dimensions that
maximize the area enclosed. Also determine what the
maximized area is.
6. Rosie O'Donnell is swimming 50 meters from the
shore. When she looks up she sees Tom Cruise
standing at water's edge 100 meters down the
beach from where Rosie is swimming. If you
have watched Rosie's television show, you know
that she has a thing for Tom Cruise. Rosie's top
speed swimming is one meter per second. Her
top speed running is 4 meters per second. What
point on the beach should she swim for in order
to minimize her time to reach Tom Cruise?
Solutions:
1. ) Step 1: Determine the derivative of f(x). You should be adept at
finding the derivative of a polynomial by now. We get f'(x) = -2x +
3. Why take the derivative? Because that is how you go about
finding stationary points.
Step 2: Solve for the derivative of f(x) to be zero. So the equation to
solve for x is: -2x + 3 = 0 You learned how to do that in algebra. The
solution is x = 1.5. But why do we have to solve this? Because we
are looking for stationary points. By definition, a stationary point is
where f'(x) = 0. And we already know from the previous step
that f'(x) = -2x + 3. Now we know that there is a stationary point for
this function at x = 1.5
Step 3: Determine the second derivative of f(x). That just means taking
the derivative of its derivative. If f(x) = -x2 + 3x - 2, then as we
already determined, f'(x) = -2x + 3. Taking the derivative of that we
get f"(x) = -2. Why take the second derivative? Because we need
to know it in order to determine whether the stationary point we've
located is a maximum or a minimum.
Step 4: Substitute the x-value of the stationary point into f"(x) and
determine if the result is positive or negative. In this case where f"(x)
= -2, it doesn't matter what you put in for x. f"(x) is always negative.
By the rule we discussed in the main text, that means the stationary
point we have located at x = 1.5 is a local maximum.
Step 5: Substitute the x-value of the stationary point into f(x). This will
determine the y value of the stationary point. So we substitute x =
1.5 into f(x) = -x2 + 3x - 2. We get y = 0.25. So the stationary point is
at (1.5, 0.25).
2.) Step 1: Determine the derivative of f(x). We get f'(x) = 3x2 3. Once again we do this because it is the first step in finding
stationary points.
Step 2: Solve for the derivative of f(x) to be zero. This gives us the xvalues of the stationary points. So the equation to solve for x is:
3x2 - 3 = 0 Divide out 3 from both sides and you have: x2 - 1 = 0 which
is a difference of squares and (by what you learned in algebra)
can be readily factored into (x + 1)(x - 1) = 0 which means that
either x = 1 or x = -1. You can reach the same solutions using
the quadratic formula as well on x2 + 0x - 1 = 0. We found these
solutions in order to locate the stationary points, which we now
have x-values of x = 1 and x = -1.
Step 3: Determine the second derivative of f(x). This is so we can
establish what's a maximum and what's a minimum. Again we
find the second derivative of f(x) by taking the derivative
of f'(x), which we already found in step 1. So we get f"(x) = 6x.
Step 4: Substitute the x-values of the stationary points into f"(x) and
determine if the results are positive or negative. When we
substitute x = 1 into f"(x) we get 6, which is positive. By the rule
established in the main text, that means that this stationary point
is a local minimum. When we substitute x = -1 into f"(x) we get 6, which is negative. By the same rule, that means that this
stationary point is a local maximum.
Step 5: Substitute the x-values of the stationary points into f(x). This
will determine the y-values of each of the stationary points. We
get f(1) = -1 and f(-1) = 3 So that means we have a local
maximum at (-1, 3) and a local minimum at (1, -1).
3.) Step 1: Determine the derivative of f(x). We do this for the same
reason we did in with the other two functions. In this case you have
to apply either the quotient rule or the chain rule in order to find
the derivative of f(x). The method using the quotient rule is pretty
obvious. Can you figure out how to take the derivative using the
chain rule? Hint: Break f(x) into a composite of
1 g(x) = x and another function. Either method of taking the derivative
yields the same result: -2x + 2 f'(x) = (x2 - 2x + 2)2
Step 2: Solve for the derivative of f(x) to be zero. Remember
that f'(x) is a quotient, and quotients can be zero only at x-values
where the numerator is zero and the denominator is not zero. The
numerator, in this case, is -2x + 2. It is zero only at x = 1. You still
have to substitute x = 1 into the denominator to make sure the
denominator doesn't become zero there. And indeed it does not.
So x = 1 is the one stationary point this function has.
Step 3: Determine the second derivative of f(x). This means taking the
derivative of f'(x), which, as you saw above, is a quotient of two
polynomials. You will definitely have to use the quotient rule to do
this. You can also make good use of the chain rule to find the
derivative of the denominator of f'(x). In fact, lets do that and find
the derivative of the denominator before we continue. We
have d(x) = (x2 - 2x + 2)2. This is the composite of g(x) =
x2 and h(x) = x2 - 2x + 2.
Taking the derivatives of both of those we have g'(x) = 2x and h'(x) =
2x - 2. When we apply the chain rule to the composite, g(h(x)) we
get d'(x) = g'(h(x))h'(x) = 2h(x)h'(x) = 2(x2 - 2x + 2)(2x - 2) Now that
we know the derivative of the denominator of f'(x), we can go on
and apply the quotient rule to find the derivative of f'(x). By the rule
we have: d(x)n'(x) - n(x)d'(x) f"(x) = = d2(x) (x2 - 2x + 2)2(-2) - (-2x +
2)(2)(x2 - 2x + 2)(2x - 2) = (x2 - 2x + 2)4 -2(x2 - 2x + 2)2 - 2(-2x + 2)2(x2 - 2x
+ 2) (x2 - 2x + 2)4
Step 4: Substitute the x-value of the stationary point into f"(x) and
determine if the result is positive or negative. Looks pretty nasty,
this f"(x), doesn't it? But we only have to know what its value is at x
= 1. Observe that the right-hand half of the numerator to this thing is
multiplied by -2x + 2, which is zero at x = 1. So we can ignore the
entire right-hand half of the numerator to f"(x) (for the purpose of
evaluating it at x = 1 only) and we find that f"(1) = -2, which is
negative. Hence, the stationary point at x = 1 is a local maximum.
Step 5: Substitute the x-value of the stationary point into f(x). This will
give you the y-value of the stationary point. The original function
again is: f(x) = x2 - 2x + 2 The x-value of the stationary point we
determined to be at x = 1. You can quickly find that f(1) = 1. So
the stationary point is at (1, 1), and it is a local maximum.
4.) Step 1: Turn what you know into functions. You had a demand
function that related the number of items sold per month, n, to
the price, x, you charged: n(x) = 40000 - 800x You also had a
monthly fixed cost of $1000, and a per item cost of $2. So your
cost function will be c(n) = 1000 + 2n where n is the number sold
(which we can determine from the price, x by the demand
function, n(x).
Step 2: Find the revenue function. Your revenue will always be price
times the number sold, so as a function of price, your revenue is:
r(x) = x n(x) = 40000x - 800x2
Step 3: Find the profit function. Profit is revenue minus cost:
p(x) = r(x) - c(n(x)) = 40000x - 800x2 - (1000 + 80000 - 1600x) p(x) = 800x2 + 41600x - 81000 We obtained the above equation by
substituting in the expressions we already have
for r(x), c(n), and n(x).
Step 4: Take the derivative. To maximize p(x) we need to
find p'(x): p'(x) = -1600x + 41600
Step 5: Maximize the profit. We find the x that maximizes p(x) by
finding where p'(x) is zero. You should have no trouble solving
this one, and you should get x = $26. You can find the actual
monthly profit, revenue, cost, and number sold by substituting
this x back into the
functions, p(x), r(x), c(n(x)), and n(x) respectively. You can
also check your answer to be sure it is a maximum (and not a
minimum) by finding p"(x) and making sure that at your
solution-x it is negative.
5.) Step 1: Identify the variables. The variable to be optimized is area, A. The
rectangle has height, h, and width, w. The semicircle has a radius, r. The
perimeter, P, is fixed by the problem at 3 meters.
Step 2: Find the relationships among the variables. The semicircle must span
the width of the rectangle. Hence its diameter must be the same as the
width. And diameter is twice radius. Hence 2r = w The perimeter, P, which
is a constant and not a variable, is the sum of the three sides plus the arclength of the semicircle. Two of the sides are of length h, the other is of
length w. And the arc-length of a semicircle is π times its radius. So P = 3
meters = 2h + w + πr Finally, area, A, is the sum of the area of the
rectangle and the area of the semicircle. Using familiar area formulas we
get 1 A = hw + πr2 2
Step 3: Substitute until you have the variable to be optimized as a function of
just one of the other variables. The variable to be optimized is area, A. You
have area shown above as a function of three other variables. You have
to get that down to a single variable. First, we can eliminate w because
we have determined that 2r = w. So the perimeter equation becomes P
= 3 meters = 2h + 2r + πr and the area equation becomes 1 A = 2hr + πr2 2
Since P is a constant and not a variable, we can use the perimeter
equation to eliminate another variable, say h. That is 2h = P - 2r - πr
Substituting that back into the area equation, you get 1 A = (P - 2r - πr)r +
πr2 2. This gives us the variable to be optimized, A, in terms of just one of
the other variables.
Step 4: Simplify. Simply multiplying out and gathering like terms simplifies the
above equation and makes it easier to deal with. I get : A = rP - (2 + π)r2 2
Step 5: Take the derivative and set it to zero. This step is at the
heart of all optimization problems. You must take the
derivative of the variable to be optimized with respect to the
one variable you have remaining. In this case, that means
taking the derivative of area, A, with respect to radius, r.
dA A' = = P - (4 + π)r dr And now we set that derivative equal to
zero. 0 = P - (4 + π)r
Step 6: Solve for the remaining variable. The one variable
remaining is radius, r. We have a simple equation for it here
that is easily solved.
P 3 meters r = = = 0.420074365 meters 4 + π 4 + π
Step 7: Substitute back to find the variable(s) you eliminated
earlier. Here we have
2P 6 meters w = 2r = = = 0.84014873 meters 4 + π 4 + π and 2P πP 2h
= P - 2r - πr = P - - 4 + π 4 + π If you put this all over a common
denominator, you get 4P + πP - 2P - πP 2P 2h = = = w 4 + π 4 +
π or w h = = r = 0.420074365 meters 2
Step 8: Determine what the optimized variable is equal to at the
point of optimization. You just solved for all the independent
variables to optimize the problem. In this case we solved for
height, width, and radius. You have actual numbers for them
now. Recall back in step 3 where you had a formula for area,
A, based upon these variables? Use it now and plug in the
numbers to get the actual area. A = hw + πr2 = 0.63011155
meters2 2 And now you are done.
Summary: The main steps all optimization problems are that you
first determine the independent variables (height, width, and
radius in this case) and the variable to be optimized (area in
this case). Then you find all the relationships you can among
them. You must, in particular, find an equation that relates
the variable to be optimized (area) to the independent
variables (height, width, and radius). Now you have to use
the relationships you have identified among the
independent variables to eliminate all but one of them (we
whittled it down to just radius, but you could have taken it
down to either of the other two, just as long as you only have
one remaining independent variable when you're done).
Then the crucial step is to write the variable to be optimized
(area) in terms of the one remaining independent variable
(radius). Take the derivative of that equation, and set it to
zero. You are now in a position to solve for that one
remaining independent variable (radius). Finally, use the
relationships among the independent variables to determine
the solutions for the ones you eliminated (height and width).
Then plug them all back into the equation for the variable to
be optimized (area) to determine its value as well.
Review these steps carefully and there will be no optimization
problem that can stump you.
6.) Step 1: Set up the problem. How are you going to identify the
point on the beach to which she swims? Make a diagram.
The beach is a straight line. Draw in Rosie's position in the
water and Tom's position on the straight line. Now draw a line
perpendicular to the beach that passes through Rosie. Where
the beach and the perpendicular intersect is the origin (this is
not the only way to set it up, but this is the way I'll do it here).
The point on the beach to which Rosie swims is x meters down
the beach from this intersection. We need to solve for x.
Step 2: What is Rosie's running time? If she swims to a position x
meters down the beach and Tom is 100 meters down the
beach, then she has 100 - x meters left to run in order to get
to him. If she runs at 4 meters per second, then her running
time, tr, is given by tr = (100 – x)/ 4
Step 2: What is Rosie's swimming time? If she swims to a point that
is x meters down the beach from where she is, then her swimpath is the hypotenuse of a right triangle. Use the
Pythagorean distance formula to set this up. One leg of the
triangle is given in the problem as 50 meters long. The other is
x meters long. So her swim distance is √(2500 + x2 )That means
her swimming time, ts, swimming at 1 meter per second, is
given by ts = √(2500 + x2 )/1
Step 3: What is Rosie's total transit time, t? Clearly it is the sum of her
running time, tr, and her swimming time, ts.t = √(2500 + x2 )+ 25 – x/
4
Step 4: Take the derivative of Rosie's transit time with respect to x. That
is, find dt/dx. When you set dt/dx to zero, you will be able to
solve for the x that minimizes t. You will have to apply the chain
rule in order to take this derivative. dt/dx = 0 = x / √(2500 + x2 )-(1/
4)
Step 5: Solve for x. You do this by multiplying the above equation
through by the radical 0 = x – (1/4)√(2500 + x2 ) Add one fourth of
the radical to both sides x = (1/4)√(2500 + x2 ). Multiply by 4, then
square both sides: 16 x2 = 2500 + x2 . 15 x2 = 2500. x2 = 2500/15. x=
50 / √15 = = 12.090994449 meters. So Rosie must swim to a point that
is 12.090994449 meters down the beach (toward Tom Cruise) from
where she is swimming. Note that Tom could have been 1000
meters down the beach (or any other distance greater than
12.090994449 meters) and this problem would still have come out
with precisely the same solution.
Now that you know how to do this one, see if you can do the following
variation on it by yourself. Rosie is still swimming 50 meters off shore.
But this time Tom Cruise appears on the deck of a beach house
that is 100 meters down the beach and 80 meters back from the
beach. Rosie can still swim 1 meter per second and run 4 meters
per second. At what point on the beach should she swim for in
order to minimize her transit time? Just go as far as to set the
minimization equation up on this one. Don't bother to solve for x
unless you're in the mood for solving a fourth degree polynomial.